In recent years, the public, industry and government at all levels have become acutely aware of the need to manage the nation's water resources more efficiently. Further, society is now demanding that cleaner streams, rivers and lakes be provided free from pollutants that might interfere with the best use of these resources. Accordingly, many new laws have been passed that prohibit the discharge of water into streams unless or until it is free from certain pollutants. As a result mine water or mine drainage is one of the more serious environmental problems facing the coal industry today. This is particularly true as it relates to long established underground operations (Crickmer, Douglas F., Zegeer, David A., Elements of Practical Coal Mining, The Society of Mining Engineers of the American Institute of Mining, Metallurgical and Petroleum Engineers, Inc., New York, N.Y., 1981, pp. 669-672).
More specifically, mining operations for coal and various ores involve the in situ exposure of pyrite and marcasite containing geologic strata to the atmosphere. These iron sulfide compounds undergo oxidation upon contact with the oxygen and water that is plentiful in the environment. This oxidation produces highly acid water enriched with various heavy metals.
In addition to pyrite and marcasite present in the geologic strata, pyrite and marcasite is often a component of the mined ore product. Upon further treatment of the ore for product purification purposes this additional pyrite and marcasite is separated and treated as a waste product. This waste product is subject to oxidation as described above and consequently also produces acid solutions enriched with heavy metals. As already indicated, both the acid solutions and heavy metals are now recognized as very significant environmental pollutants that must be controlled. Up until the development of the present invention, however, there has been no reliable, long term, economic technique to reduce or prevent the oxidation of the pyrite and marcasite.
To date, there have been three approaches utilized to treat pyrite/marcasite rich material in order to control oxidation. In the first approach, neutralizing agents are utilized to treat acid drainages produced from the oxidation. Unfortunately, this approach only treats the symptoms of the oxidation and simply does not address the cause. Further, the approach is very costly. Additionally, it only represents a short term solution as it is not feasible to periodically return to the site to retreat the drainages over an extended period of time.
The second approach involves the utilization of detergents to kill sulfur oxidizing bacteria. Unfortunately, this approach is also only a relatively short term solution lasting from six months to a year. Further, it is also only suited to certain field conditions and thus has limited applications. Still further, utilization of some of the detergents also effects the environment adversely and accordingly, the impact of this procedure must be closely monitored.
The third and final approach commonly utilized is physical encapsulation of the pyrite/marcasite containing waste products. More particularly, clay liners, plastic liners and blacktop liners may be utilized to prevent oxygen and water from reaching the pyrite and marcasite. Unfortunately, this approach is very expensive. Additionally, the liners are subject to cracking and the cost to repair cracked liners is prohibitive. Accordingly, this approach is rarely used by mine operators.
A fourth approach is proposed in Flynn U.S. Pat. No. 3,443,882. In this patent earth strata; rocks, mine tailings, gob piles and the like are treated to prevent oxidation of iron sulfide compounds such as pyrite and marcasite. More specifically, Flynn teaches the application of a phosphate coating on the pyrite and marcasite by mixing the materials with an inorganic phosphate such as naturally occurring calcium phosphate. This phosphate is disclosed as gradually converting the iron sulfide (FeS.sub.2) to calcium sulfate (CaSO.sub.4), and vivianite (Fe.sub.3 (PO.sub.4).sub.2).
Experiments have, however, unfortunately shown that if one applies calcium phosphate to pyrite/marcasite in nature, the calcium phosphate actually becomes coated through a well-known reaction called metal surface adsorption. More specifically, as the pyrite and marcasite are oxidized through contact with oxygen and water, ferrous sulfate (FeSO.sub.4) is generated. This is a very soluble salt that rapidly coats the pulverized calcium phosphate rendering it inactive. Accordingly, it has been found that the metal surface adsorption not only prevents the production of vivianite as suggested by Flynn but it also makes the Flynn approach inoperative.
In an effort to avoid the coating of the phosphate itself, Flynn recommends in the patent the application of sulfuric acid. However, it should be appreciated that sulfuric acid causes very significant damage to ground water supplies by dissolving more heavy metals. Accordingly, this proposed solution to the problem is simply environmentally unacceptable. Additionally, the Flynn patent suggests the utilization of chlorine or bromine gas to kill the sulfur oxidizing bacteria. At best, such a procedure would only be effective for a few months before additional treatments would be required. Furthermore, it is unlikely that the utilization of such highly toxic gasses as chlorine or bromine would ever be approved by the government. As a result of these and other shortcomings, the approach proposed by Flynn has not been put into practice.
Following review of this background to the pyrite and marcasite oxidation problem, it should be appreciated that a need exists for a better solution. Further, the need is great. So as to have a better understanding of the extent of the problem, an average coal processing plant produces anywhere from 1 to 5 tons of finely ground pyrite per hour. Thus, this is a very significant environmental problem.